RETINOIC ACID RECEPTOR ISOFORM BETA-2 IS AN EARLY MARKER FOR ALIMENTARY-TRACT AND CENTRAL-NERVOUS-SYSTEM POSITIONAL SPECIFICATION IN THE CHICKEN

In this study I describe the distribution of one variant of retinoic acid receptor-beta (RAR-beta), the RAR-beta 2 isoform, during the stages before organogenesis of the chick embryo. Unlike the situation in older embryos, at these stages its distribution does not differ qualitatively from that of all RAR-beta transcripts. During the presomite headfold stage, RAR-beta 2 transcripts are simultaneously upregulated in two different locations. These locations define positional identities within the anlage of the chick alimentary tract and within the central nervous system (CNS). As development proceeds the transcript expression maintains its spatial restriction within those two regions. At presomite stages RAR-beta 2 transcripts are enriched within the proamnion, which contains the presumptive foregut and precardiac cells; somewhat later it is present within the foregut endoderm at the site where foregut and the lateral amniocardiac vesicles fuse to form the coelom and cardiac tube. As the foregut continues its caudal extension, RAR-beta 2 expression defines an anteroposterior boundary at the level of the pharynx within the alimentary tract. The second expression site of RAR-beta 2 mRNA first appears within the posterior neural plate at the level where Hensen's node commences its caudal regression. This boundary lies at the border between the future rhombomeres 5 and 6 within the hindbrain. Expression of RAR-beta 2 transcripts is also spatially restricted within some migrating cranial neural crest cells. The expression of RAR-beta 2 in cranial neural crest cells is consistent with what is known about the mechanisms by which cranial neural crest cell fate is determined. These data support the hypothesis that retinoids may contribute to positional specification of anteroposterior body axis, and perhaps also to the formation and identity of the developing alimentary tract and heart tube. (C) 1994 Wiley-Liss, Inc.